Transimpedance amplifiers can be used to convert an input current signal into an output voltage signal. This is useful, for example, to process current signals generated by photodetectors from the detection of light. To detect a low-level light signal a relatively large gain is needed for the transimpedance amplifier; whereas for the detection of a high-level light signal this relatively large gain can lead to a saturation of the output voltage signal from the transimpedance amplifier. Thus, there is a need to extend the dynamic range of transimpedance amplifiers so that both low-level light signals and high-level light signals can be detected since, in many cases, the exact signal level of the light is unknown prior to detection.
In the prior art, many different attempts have been made to address the need for transimpedance amplifiers with a high dynamic range (see e.g. U.S. Pat. Nos. 7,050,724; 7,092,644; 7,205,845; 7,474,978; 7,492,399 and U.S. Patent Application Publication No. 2003/0090326). Most, if not all, of the prior art attempts to extend the dynamic range for a single transimpedance amplifier have resulted in an output voltage signal which is nonlinearly related to the input current signal. This is undesirable for applications where the exact shape of the output voltage signal is important to determine characteristics of the input current signal being detected without having to revert to complex mathematical algorithms or look-up tables. Some attempts to extend the dynamic range of transimpedance amplifiers have also used cascaded low-gain amplifiers; but this compromises low-noise performance for increased dynamic range.
The present invention provides an advance in the art by providing a transimpedance amplifier system which generates a pair of output voltage signals from an input current signal using a single transimpedance amplifier. These two output voltage signals are scaled and combined in a signal linearizer to generate a high-gain output signal and a low-gain output signal with one or both of these signals being linearly related to the input current signal.
The transimpedance amplifier system of the present invention does not substantially degrade small-signal noise performance in the high-gain output signal for certain embodiments of the present invention where the high-gain output signal is not linearized. In these embodiments of the present invention, the low-gain output signal has a slightly higher noise level as compared to the high-gain output signal. This is not significant, however, since the low-gain output signal is generally used in preference to the high-gain output signal when the input current signal is relatively large.
These and other advantages of the present invention will become evident to those skilled in the art.